demitter

Demitter - Distributed Event Emitter (Pub/Sub) for Node.js

Extend your events across processes, threads, and machines. Demitter brings the familiar Node.js event API (via emittery) API to distributed systems, enabling real-time event communication between multiple Node.js processes with near-zero configuration.

🎬 See It In Action

Live auction system with multiple bidders across separate terminal processes - all synchronized in real-time

✨ Features

• 🚀 Distributed Events: Emit events across processes, threads, or machines
• ⚡ Zero Configuration: Works out of the box with sensible defaults
• 🎯 Familiar API: Extends the battle-tested sindresorhus/emittery library
• 🛡️ Type Safe: Full TypeScript support
• 📦 Lightweight: Efficient binary serialization with minimal overhead
• 🔧 Flexible: Embedded or standalone message proxy/forwarder deployment options

📦 Installation

npm install demitter
# or
pnpm install demitter
# or
yarn add demitter

🚀 Quick Start

Get distributed events running in 3 steps:

1. Start the Message Forwarder

import { createForwarder } from "demitter";

const forwarder = await createForwarder();
console.log("Forwarder ready - events can now flow between processes!");

2. Create Distributed Emitters

import { createDistributedEmitter } from "demitter";

// In Process A
const emitterA = await createDistributedEmitter();

// In Process B
const emitterB = await createDistributedEmitter();

3. Emit Events Across Processes

// Process A: Listen for events
emitterA.on("user:login", (data) => {
  console.log("User logged in:", data);
});

// Process B: Emit events that Process A will receive
emitterB.emit("user:login", { userId: 123, timestamp: Date.now() });

That's it! Events emitted in one process are automatically received by all other connected processes.

🔌 API Reference

Demitter extends the powerful emittery API with distributed capabilities. All emittery methods work exactly the same:

Core Methods

// All the familiar emittery methods work across processes:
await emitter.emit(eventName, data);
emitter.on(eventName, listener);
emitter.off(eventName, listener);
await emitter.once(eventName);
emitter.onAny(listener);
emitter.clearListeners();
// ... and many more

📖 Full API Documentation: See the complete emittery API docs for all available methods, TypeScript usage, and advanced features.

Distributed-Specific APIs

createDistributedEmitter(options?)

Creates a new distributed emitter instance.

const emitter = await createDistributedEmitter({
  xsubAddress: "tcp://localhost:5555", // Connect to forwarder
  xpubAddress: "tcp://localhost:5556", // Connect to forwarder
});

createForwarder(options?)

Creates and starts a message forwarder to enable event distribution.

const forwarder = await createForwarder({
  xsubPort: 5555, // Default port for subscribers
  xpubPort: 5556, // Default port for publishers
});

emitter.close()

Closes the distributed emitter and cleans up connections.

await emitter.close();

Events

• distributed:error: Emitted when distributed operations fail (network issues, serialization errors, etc.)

emitter.on("distributed:error", (error) => {
  console.error("Distributed operation failed:", error);
});

🎯 Use Cases

• Microservices Communication: Event-driven architecture between services
• Multi-Process Applications: Coordinate events across worker processes
• Real-Time Systems: Live updates, notifications, and state synchronization
• Distributed Logging: Centralized event collection from multiple sources
• Game Development: Real-time multiplayer game state management
• IoT Networks: Device communication and sensor data distribution

🗂️ Examples

Explore practical examples in the examples/ directory:

• basic-usage.ts - Simple distributed events setup
• error-handling.ts - Robust error handling patterns
• live_auction/ - Complete live auction system demo
  • Multi-process auction with bidders, controller, and spectator dashboard
  • Real-time terminal UIs with charts and analytics
  • Intelligent bidding strategies and conflict resolution

Running Examples

# Basic usage
pnpm run example:basic

# Error handling
pnpm run example:errors

# Live auction demo (see examples/live_auction/auction-demo/README.md)
cd examples/live_auction/auction-demo
pnpm install
pnpm run demo

🚀 Deployment Options

Embedded Forwarder

Start the forwarder within your application:

import { createForwarder, createDistributedEmitter } from "demitter";

const forwarder = await createForwarder();
const emitter = await createDistributedEmitter();

Standalone Forwarder

Run the forwarder as a separate service:

# Using the CLI
npx demitter-forwarder

# Or start with custom ports
XSUB_PORT=7777 XPUB_PORT=8888 npx demitter-forwarder

Docker Deployment

FROM node:22-alpine
RUN npm install -g demitter
EXPOSE 5555 5556
CMD ["demitter-forwarder"]

Architecture

The system consists of two main components:

• Pub/Sub Forwarder - A standalone message broker that forwards events between distributed processes
• DistributedEmitter - An extension of Emittery class in the emittery library for distributed event emission

Pub/Sub Forwarder

The forwarder acts as a central message broker using ZeroMQ's XSUB-XPUB proxy pattern.

Usage

As a global CLI command:

After installing the package globally:

npm install -g demitter
# or
pnpm install -g demitter

You can run the forwarder from anywhere:

# Start with default ports (XSUB: 5555, XPUB: 5556)
demitter-forwarder

# With custom ports
XSUB_PORT=6000 XPUB_PORT=6001 demitter-forwarder

# With debug logging
LOG_LEVEL=debug demitter-forwarder

# Show help
demitter-forwarder --help

As a local CLI command:

For projects with demitter as a dependency:

npx demitter-forwarder

As a standalone process:

# Start with default ports (XSUB: 5555, XPUB: 5556)
node src/forwarder.js

# Or using the CLI script
node src/cli.js

# With custom ports
XSUB_PORT=6000 XPUB_PORT=6001 node src/forwarder.js

# With debug logging
LOG_LEVEL=debug node src/forwarder.js

As a module:

import { createForwarder } from "./src/forwarder.js";

// Start forwarder with default configuration
const forwarder = await createForwarder();

// Custom configuration
const forwarder = await createForwarder({
  xsubPort: 6000,
  xpubPort: 6001,
  loggerOptions: { level: "debug" },
});

// Graceful shutdown
await forwarder.close();

Configuration

The forwarder can be configured using environment variables:

Variable	Default	Description
XSUB_PORT	5555	Port for XSUB socket (receives from publishers)
XPUB_PORT	5556	Port for XPUB socket (sends to subscribers)
XSUB_ADDRESS	tcp://*:${XSUB_PORT}	Full address for XSUB socket
XPUB_ADDRESS	tcp://*:${XPUB_PORT}	Full address for XPUB socket
LOG_LEVEL	info	Logging level (debug, info, warn, error)

CLI Help

demitter-forwarder --help
# or when running locally
node src/cli.js --help

License

See LICENSE file.

🤝 Contributing

I'm open to contributions but the details for contributions are not yet finalized. If you have ideas or improvements, feel free to open an issue or start a discussion before you raise a pull request.